Please solve in the picture! I know the answer is that BC is not the longest side but I need an explanation

Case : BC is the longest side

#Remember that the total degree of triangle is 180°

#If A° = 120° so B° + C° = 180°-120°

B° + C° = 60°

#From here, you already know that even the total of both of B and C is less than 120° which is the degree of A.

#If you are gonna find the length of BC, then you need to use trigonometry which will be

BC/sinA = AB/sinB = AC/sinC

#so, from here you can know that BC will be the longest one, because bigger the value, in sin, will be smaller the value